The use of electrochemical devices for determining and measuring (activity) concentration of ions in solutions is now commonplace. In the usual form, such a device consists of a ion-selective electrode such as for instance a pH-selective glass electrode which is immersed into the solution whose ion (activity) concentration is to be measured. When the ion-selective electrode is immersed into the solution, a potential difference develops between the inner and outer layers of the membrane which is related to the ionic concentrations of the solution. Essentially, the ion-selective electrode and the solution constitute a half-cell and the developing potential is called the half-cell potential of the ion-selective electrode. To measure this half-cell potential, it is necessary to connect the ion-selective electrode and the solution to an electric measuring circuit. To do this, it is necessary to bring the solution into contact with another electrode, whereby the second electrode, also called reference electrode, should develop a constant half-cell potential which is essentially unaffected by concentration changes in the test solution. Usual practice employs a substantially reversible electrode system such as calomel electrode which is surrounded by an electrolyte such as, for instance, a saturated KCl solution, which in turn contacts the test solution. The junction between test solution and saturated KCl solution is called liquid junction. Generally one provides a constriction in the liquid path between two half cells which reduces the liquid flow between the half cells to a minimum, yet permits electrical conduction through the adjoining liquids.
Ion-selective electrodes obey the Nernst Equation: EQU E=E.sub.0 +(RT/nF)log a.sub.i
wherein:
E=the single electrode potential;
E.sub.0 =the reference electrode potential;
R=the gas constant;
T=the absolute temperature;
n=the valence of the ion;
F=Fraday's constant; and
a.sub.i =the ion activity in the solution.
Any change in the activity of the measured species in solution causes a change in the measured potential which can be related to the (activity) concentration of the unknown specimen by proper calibration. Ion-selective electrodes are available for anions (e.g. F-, Cl-, Br-, etc) and cations (e.g. H+, Na+, K+ etc.) They are also available for some divalent ions like Ca++.
Ion-selective electrodes are classified into four broad groups:
1. Glass electrodes, e.g. H+, Na+;
2. Pressed pellet or single crystal electrodes, e.g. F-, Br+;
3. Gas sensing electrodes Ammonia, CO.sub.2.
4. Liquid membrane electrodes, e.g. K+, Ca++, etc.
The present invention deals mostly with the liquid membrane electrodes. These electrodes were originally made by dissolving the active ingredient (e.g. for K+, Valinomycin) in an appropriate solvent and impregnating a filter paper with this solution. Ideally, the active ingredient is practically insoluble in water and the solvent selected also has minimum solubility in water. This type of electrode is clumsy to make and has a fairly short useful life. Some electrodes were made with a built-in reservior of the "active solution". These electrodes gave a somewhat longer life but were difficult to assemble and were not popular because of the obvious drawbacks.
The next development in liquid membrane electrodes was the making of a polymeric membrane. These were made by dissolving a polymer such as polyvinylchloride (PVC) in tetrahydrofuran (THF) and then adding the active ingredient such as Valinomycin to the mixture together with a plasticizer (in this case, diethyl hexylsebacate) which is a solvent for the active ingredient. After thoroughly mixing, the mixture is evaporated in a flat container to drive out the solvent THF. This leaves a film of PVC which can be easily peeled off from the container. Ideally this film has a thickness of 6-10 mils.
Dip electrodes were made by "gluing" a small piece of this membrane to the end of an open tube and using Ag/AgCl wire with KCl filling as the internal reference electrode. This type of electrode works well, has extended life and was easy to use.
The membranes in film form were also made into flow-thru electrodes involving a fairly complicated electrode design. These flow-thru electrodes are difficult to assemble properly and, once assembled, the life of the electrode is very unpredictable. U.S. Pat. Nos. 4,233,136 and 4,314,895 describe a "flow-through", liquid membrane electrode and a method of making the electrode. The flow-through electrode comprises a tube in which a portion of the wall comprises a membrane containing a liquid phase ion exchange material for the electrode. The membrane is integrally sealed to the wall of the flow-through tube. The method of making the electrode comprising the steps of dissolving an organic plastic matrix material in a volatile solvent and then mixing a non-volatile solvent-plasticizer and an ion exchange material (in case of K+ it is a neutral carrier complex), which is soluble in the plasticizer, with the plastic material and the volatile solvent. The solution thus obtained is cast on a surface to form a membrane as the volatile solvent is evaporated. The membrane is attached to a tube of organic plastic material by contacting the tube with a volatile solvent common for the membrane and the tube and abutting the membrane material against the tube. As the solvent evaporates, the tube and membrane are integrally joined. In a particular embodiment, an opening is formed in the tube to receive the membrane. A mandrel is inserted within the tube and across the opening. The membrane is then formed on the mandrel contacting the tube edges at the opening and the volatile solvent in the membrane contacts the tube edges thereby resulting in the joinder of the membrane to the tube as the volatile solvent evaporates.
In a particular embodiment of the above-described flow-through electrode for detecting potassium ions, the matrix material is polyvinylchloride, the ion-exchange material is valinomycin, the non-volatile solvent comprises 2-nitro-p-cymene and the volatile solvent is a tetrahydrofuran. The membrane thickness is preferably in the range of from 8-12 mils, the polyvinylchoride matrix material comprises from 8-26%, preferably from 12-20 %, by weight of the membrane. The patentees report that an electrode assembly according to the invention supports and seals a liquid membrane integrally with the wall of the tube thereby permitting the construction of a linear flow-through liquid membrane electrode. The smooth linear flow path avoids turbulence and eddy currents as well as mechanical discontinuities which can trap portions of the liquid sample being tested and permits a more accurate, rapid and reliable response. The use of tubing allows a small diameter flow path minimizing the amount of sample required. The matrix support of the ion exchange material and the linear flow path eliminate the danger of rupturing the liquid membrane. The fabrication of the membrane to the flow-through tube is simple and convenient.
Good performance and long life of these ion-selective electrodes requires the proper plasticizer in the membrane in a fairly large quantity and, of course, a large amount of the active ingredient which is held in the membrane with the plasticizer. One of the problems associated with the flow-through electrodes described above is the limited surface area possible if the membrane is to be structurally stable. The membrane is relatively delicate compared to the tube wall. While the membrane is bonded to the tube wall, nevertheless, the size of the "interruption" in the tube wall described by the patentees must be limited to a small area into which the membrane is relatively self-supporting. If the active area of the membrane is too great, it will cease to be self-supporting and will collapse or, at the least, be subject to rupture or breaking with relatively little force. The limited size of the available active area, therefore, severely limits the amount of plasticizer that can be maintained in the membrane and, therefore, also the amount of dissolved active ingredient. As a result, many of these structurally stable electrodes of the above described type have a relatively short life under normal use conditions.